[utils] Multiple changes to base_n()

1. Renamed to encode_base_n()
2. Allow tables longer than 62 characters
3. Raise ValueError instead of AssertionError for invalid input data
4. Return the first character in the table instead of '0' for number 0
5. Add tests

youtube_dl/utils.py

@@ -2621,15 +2621,17 @@ def ohdave_rsa_encrypt(data, exponent, modulus):
     return '%x' % encrypted
 
 
-def base_n(num, n, table=None):
-    if num == 0:
-        return '0'
-
+def encode_base_n(num, n, table=None):
     FULL_TABLE = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
-    assert n <= len(FULL_TABLE)
     if not table:
         table = FULL_TABLE[:n]
 
+    if n > len(table):
+        raise ValueError('base %d exceeds table length %d' % (n, len(table)))
+
+    if num == 0:
+        return table[0]
+
     ret = ''
     while num:
         ret = table[num % n] + ret
@@ -2649,7 +2651,7 @@ def decode_packed_codes(code):
 
     while count:
         count -= 1
-        base_n_count = base_n(count, base)
+        base_n_count = encode_base_n(count, base)
         symbol_table[base_n_count] = symbols[count] or base_n_count
 
     return re.sub(